The invention relates to an array of ultrasound transducers which each comprise a substrate, a membrane, a first electrode, a piezoelectric layer, and a second electrode, said substrate comprising at least one opening which adjoins the membrane at one side. The invention further relates to an ultrasound transducer and to a method of manufacturing an ultrasound transducer.
The generation of ultrasonic sound takes place by purely mechanical means or by means of electroacoustic transducers which utilize the magnetostrictive or piezoelectric effect. Since ultrasound can be easily realized technically nowadays, it is widely used. Thus ultrasound is used for generating images in medical diagnostics or in non-destructive material testing.
The electroacoustical transducers used most widely are based on the piezoelectric effect. In practice, one-dimensional or two-dimensional array systems are mostly used in addition to single-transducer systems. Two-dimensional array systems are particularly interesting for the display of three-dimensional images.
The excitation of the piezoelectric elements in acoustic transducers takes place either in an AC field with a frequency of a few kHz up to several MHz or, in particular in image generation, by short oscillation bursts with a basic frequency of a few MHz and relative bandwidths of up to 100%. The excursion of the piezoelectric elements in the field direction generates a continuous or pulsatory ultrasound wave in the coupled medium such as, for example, water or biological tissue. The reflections changing in dependence on the tissue density and the throughput times changing with the path length are utilized for image generation in medical diagnostics.
Piezoelectric ultrasound transducers are manufactured from piezoelectric ceramic blocks nowadays. Piezoelectric ceramic blocks of exactly defined dimensions and with electrodes on the upper and lower sides of the blocks are mounted in an array in a mounting preform.
It is a disadvantage of these conventional systems that the control circuits must be separately constructed and cannot be integrated into the system.
Less expensive ultrasound transducer array systems are formed by the so-called piezoelectric micromachined ultrasound transducers (PMUT). The arrays of piezoelectric ultrasound transducers are provided directly on silicon. Such an ultrasound transducer comprises besides a silicon substrate a membrane on which a first electrode, a piezoelectric layer, and a second electrode are provided. The membrane can be obtained simply through etching away of the silicon so that an opening is created. The change in length of the piezoelectric element excites the membrane into oscillation. To manufacture an array of such ultrasound transducers, several openings are generated for the creation of several membranes on one silicon substrate.
Such an ultrasound transducer is known from U.S. Pat. No. 5,956,292. The material used for the piezoelectric layer is a ceramic material such as, for example, ZnO, AlN, LiNbO4, PbZrxTi1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61), BaTiO3 or SrTiO3.
For broadband frequency applications, however, materials with a particularly high piezoelectric coupling coefficient k are required.
It is accordingly an object of the present invention to provide an improved ultrasound transducer with a piezoelectric layer which has a high piezoelectric coupling coefficient k.
This object is achieved by means of an array of ultrasound transducers which each comprise a substrate, a membrane, a first electrode, a piezoelectric layer, and a second electrode, said substrate comprising at least one opening which adjoins the membrane at one side, wherein said piezoelectric layer is a textured layer.
A clear increase in the piezoelectric coupling coefficient k is achieved through texturing of the piezoelectric layer as compared with a polycrystalline piezoelectric layer.
It is preferred that the piezoelectric layer is a monocrystalline layer.
An absolutely ideal texturized pattern is obtained if the piezoelectric layer is formed by a monocrystalline layer.
It is furthermore preferred that the material of one electrode is textured.
The manufacture of a piezoelectric layer on a textured electrode renders it possible to provide the material of the piezoelectric layer in a textured manner.
It is particularly highly preferred that the piezoelectric layer comprises a material chosen from the group Pb(Zn⅓Nb⅔)O3xe2x80x94PbTiO3, Pb(Mg⅓Nb⅔)O3xe2x80x94PbTiO3, Pb(Ni⅓Nb⅔)O3xe2x80x94PbTiO3, Pb(Sc⅓Nb⅔)O3xe2x80x94PbTiO3, Pb(Zn⅓Nb⅔)1xe2x88x92Xxe2x88x92Y(Mnxc2xdNbxc2xd)xTiyO3 (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61), Pb(Inxc2xdNbxc2xd)O3xe2x80x94PbTiO3, Sr3TaGa3Si2O14, K(Sr1xe2x88x92xBax)2Nb5O15 (0xe2x89xa6xxe2x89xa61), Na(Sr1xe2x88x92xBax)2Nb5O15 (0xe2x89xa6xxe2x89xa61), BaTiO3, (K1xe2x88x92xNax) NbO3 (0xe2x89xa6xxe2x89xa61), (Bi,Na,K, Pb,Ba)TiO3, (Bi,Na)TiO3, Bi7Ti4NbO21, (K1xe2x88x92x Nax)NbO3xe2x88x92(Bi,Na,K,Pb,Ba) TiO3 (0xe2x89xa6xxe2x89xa61), a(BixNa1xe2x88x92x)TiO3xe2x88x92b(KNbO3xe2x88x92c)xc2xd(Bi2O3xe2x80x94Sc2O3) (0xe2x89xa6xxe2x89xa61, a+b+c=1), (BaaSrbCac) TixZr1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61, a+b+c=1), (BaaSrbLac)Bi4Ti4O15 (a+b+C=1), Bi4Ti3O12, LiNbO3, La3Ga5.5 Nb0.5O14, La3Ga5SiO14, La3Ga5.5Ta0.5O14 and PbZrxTi1xe2x88x92xO3 (0xe2x89xa6xxe2x89xa61) with and without dopants of La, Mn, Fe, Sb, Sr, Ni or combinations of these dopants.
It is advantageous that a barrier layer is provided on that side of the substrate which faces the piezoelectric layer.
This barrier layer prevents chemical interactions between the membrane and the layer present thereon during processing of the transducer.
It is preferred that the first and the second electrode are disposed at opposite ends of the piezoelectric layer.
By placing the electrodes at opposite ends of the piezoelectric layer a poled operation of the piezoelectric layer is enabled.
It is also preferred that the electrodes are disposed on the same surface of the piezoelectric layer.
It is furthermore preferred that at least one further electrode is disposed between the first and second electrodes.
The electrical impedance of the transducer is decreased by this electrodes arrangement.
It is advantageous that at least two electrodes are of the same polarity and at least one electrode is of opposite polarity, wherein electrodes of the same polarity are coupled in parallel.
By this means the electrical impedance of the transducer is reduced.
It is preferred that the electrodes form concentric rings.
The invention also relates to an array of ultrasound transducers which each comprise a substrate, a membrane, a first electrode, a piezoelectric layer, and a second electrode, said substrate comprising at least one opening which adjoins the membrane at one side, characterized in that a barrier layer is provided on that side of the membrane which faces the piezoelectric layer.
This barrier layer prevents chemical interactions between the membrane and the layer present thereon during processing of the transducer.
It is preferred that one of the electrodes is a textured electrode formed on the barrier layer.
It is also preferred that the barrier layer has a bending stiffness which is less than that of the membrane layer.
It is advantageous that the distance between the neutral plane of a transducer and the piezoelectric layer is larger than the thickness of the piezoelectric layer.
By this measures the effective coupling coefficient is enhanced.
It is also advantageous that the barrier layer is thicker than the piezoelectric layer.
The invention further relates to an ultrasound transducer which comprises a substrate, a membrane, a first electrode, a piezoelectric layer, and a second electrode, the substrate comprising at least one opening which adjoins the membrane at one side, wherein said piezoelectric layer is a textured layer.
The invention also relates to a method of manufacturing an array of ultrasound transducers which each comprise a substrate, a membrane, a first electrode, a piezoelectric layer, and a second electrode, the substrate comprising at least one opening which adjoins the membrane at one side, wherein said piezoelectric layer is manufactured in a textured manner.